With the advent of offshore drilling in order to produce petroleum products from production zones located beneath bodies of water, it has become desirable to provide casing hanger systems that are adapted to be positioned at or near the bottom of the body of water. These hanger systems are typically known as mudline suspension systems. Mudline suspension systems use an outermost casing hanger which suspends a coaxial series of concentric casing strings such that their combined weight is suspended at the mudline. This allows the drilling rig to operate in deeper than normal waters, and provides for disconnection and removal of equipment above the mudline when the drilling rig moves from one drilling location to another drilling location or when the driller moves away from the well and subsequently re-establishes a well drilling connection when it is desirable to continue drilling operations. By locating suspension systems at or near the ocean floor, a temporarily abandoned well or capped well does not present an obstruction that typically interferes with the marine environment. Such suspension systems also enable the driller to complete wells by means of an ocean floor completion or extend the casing to the surface for completion on a drilling ship or platform and subsequently lends a degree of flexibility in completion systems that renders such casing hanger systems desirable.
In mudline suspension systems, concentric casing strings are hung and cemented in place as the drilling progresses to increasing depths. Typical diameters for various casing strings are 30", 20", 16", 133/8", 95/8" and 7". When drilling a subsea well from a fixed platform, it is desirable to support the casing weights from the mudline with a blowout preventer located at the platform. Risers extend from the blowout preventer to the support location and are of substantially the same size as the casing string itself. The riser may be several hundred feet long and is made up of successive riser pipes whose adjacent ends are connected at the water's surface as the riser is lowered into position, or disconnected as the riser is raised. Each of a plurality of inner casing strings is lowered into a bore drilled in the ocean floor by means of a hanger connected to the riser. When the hanger is landed within the hanger from which the next outer casing string is suspended, cement is pumped and circulated down through the flowbore of the riser, hanger and suspended string, around the terminal end of the string, and up into the annulus around the suspended string, to anchor it in place. It is necessary that the cement pass between the adjacent hangers of the inner and outer casing strings. When the well has been tested, the riser may be retrieved, and the hangers at the upper ends of the casing strings capped or closed off at the ocean floor to permit the drilling rig to be moved to another location. When it is desired to complete the well for production purposes, the cap is removed and risers are lowered into connection with at least the innermost suspended casing strings to tie them back to a production platform at the surface of the water. The successive hangers are supported on one another so that the load of all of the hangers and the casing supported from the hangers is supported by a seat in the bore of the outermost casing hanger.
The casing hangers are connected to the upper ends of successively smaller diameter casing strings which are adapted to be lowered into and landed within the bore of a casing hanger which is connected to an outermost casing string at the mudline in order to suspend the strings within the outermost casing of the wellbore. The annular space, commonly called an annulus, between an outer casing string and the next inner casing string permits cement returns to circulate therethrough as the string is cemented within the wellbore, or adapted to be closed off, when the casing has been cemented. Casing strings of a large diameter, as for example, 16", 20" or 30", have sufficient annular space to allow the use of solid hangers, normally in the form of an annular landing shoulder on the outer casing hanger, which in turn, suspends an inner casing hanger having an annular support shoulder. Such shoulders typically have a bypass or flute therethrough to connect the annulus above and below the hangers for the circulation of cement returns.
Casing strings of a smaller diameter severely limit the annular space by which to support the next inner-casing hanger and also allow adequate flow passages therebetween for the circulation of cement returns. Because the annular spaces between the inner-most casing strings are much smaller, typically the hangers are provided with support members which are withdrawn or retracted until the string is lowered into the wellbore to dispose the support members opposite the landing member on the next outer hanger. Thus, in smaller strings, there is more limited annular space available for support and the support must be arranged in such a way as to permit flow through the annular space to facilitate cementing operations.
One prior art type of hanger includes a support member having a circumferentially split ring which is contractible within a recess in the outer surface of the inner hanger body as the string is being lowered, and which has a landing surface on its lower end which, when the string has been so lowered, expands outwardly into a supported position on a landing member in the form of an upwardly facing seat extending radially inward from the bore of the outer hanger of the next outer casing string. However, in order to support the weight of the casing string, the expandable rings must have relatively large support surfaces, which of course require landing surfaces on the next outer hangers of equally large radial extent. As a consequence, in order for the hanger bodies to be thick enough to withstand pressure differences between the casing strings, it has heretofore been thought necessary, in apparatus of this type, to vertically stagger the expandable support ring and landing surface on at least some of the hangers. This in turn has increased the height of each such hanger and thus the size and cost of the suspension system.
In another system of the prior art, the inner casing hanger with its string of casing includes a diametrically compressible collet which is urged outwardly. The collet includes specially-shaped support shoulders extending outwardly which engage grooves in the previously-set outer hanger. The inner casing hanger then rests on this collet. Means such as shear pins are required to carry the collet on the inner casing hanger at least until it enters the casing below the blowout preventer and sometimes to pull the collet down until it reaches the support elevation. Other systems use the load support shoulder to push the collet down after means are provided to constrain the collet until it enters the outer casing string.
In another embodiment of the prior art, the inner and outer casing strings are connected together by means of a resilient expandable and contractible locking support element mounted on the inner casing hanger which is biased radially outwardly but free to expand and contract radially until it engages a mating profile in the outer casing string. After engagement, a releasable means permits the locking support element to move axially with respect to the inner casing hanger to a locked expanded position and support the weight of the inner casing string on the outer casing string. By providing two or more coacting load bearing shoulders between the inner casing string and the locking support element and two or more coacting load bearing shoulders between the outer casing string and the locking support element, a greater area of load bearing surfaces is provided in a limited annular space. Longitudinal slots are provided in the locking support element for the by-pass of fluid flow.
The above prior art designs present a considerable restriction to flow during cementing. The arrangement of these prior art suspension systems, such as a contractible split ring, compressible collet, or other contractible locking support element, forces the fluid to flow through a tortuous path through expensive milled slots. Further, as wells approach greater depths, the innermost hangers must carry increased load thus requiring larger support surfaces thereby further reducing available space.
Typically, in the above prior art designs, the load carrying member also serves as the triggering mechanism. This results in these members having to resist considerable bending stresses, a condition which precludes manufacturing the suspension system by casting. Castings invariably have some porosity which makes their resistance to bending less reliable than if the parts in the suspension system are forged or machined from bar stock.
These prior art suspension members also occupy a larger portion of the latch profile such that debris and drilling mud filter cake can accumulate in the latch profile and greatly impair the latching process. The support members which enter the profiles of the previously run outer casing hanger must be fully engageable despite any mud that may have previously accumulated in the profiles. The restriction of the annular space between the innermost casing hangers further encourages the accumulation of debris and drilling mud in the profiles.
The limited annular space between casing strings of a relatively smaller diameter is of particular concern when deep wells are drilled which deviate from vertical. A casing hanger for suspending an inner casing string of a relatively small diameter may suspend 10,000 to 15,000 feet of casing weighing approximately one million pounds. Previously, in vertical wells, the smaller casing strings were often rotated to assist the cement in completely filling the annulus. However, in strings of 10,000 to 15,000 feet, the inner casing string cannot be rotated to assist in causing the cement to fill the annulus in a deviated well. Although not recommended, many operators reciprocate the inner casing string to assist in completely filling the annulus with cement. To allow reciprocation, it is necessary to have a casing hanger which does not require rotation to suspend the inner casing hanger within the outer casing hanger. In particular, the inner casing hanger must not be latched down or locked down such that the casing string may be reciprocated. Further, the act of suspending the inner casing hanger within the outer casing hanger must be repeatable to allow for the initial suspension, the subsequent reciprocation, and then a final suspension after the cementing operation has been completed.
The present invention overcomes the deficiencies of the prior art.